Dark Stars, Black Holes, Bright Galaxies

"Hearts of Darkness"

Galaxies

We live in a spiral galaxy. Our Solar System resides about three
quarters of the way out from the centre of our Galaxy, or "Milky Way",
in a spiral arm consisting of gas and young stars. However, galaxies
exist in several different forms. Elliptical galaxies are large,
round, aggregates of predominantly old stars. Spirals, like our
Galaxy, possess disks with catherine wheel-like arms that are the sites
of ongoing star formation.

An infrared image of our Galaxy taken by the Diffuse Infrared
Background Experiment (DIRBE) instrument on the NASA Cosmic Background
Explorer (COBE) satellite. The galactic plane runs horizontally along
the middle of the image. Absorption by interstellar dust is minimized at
infrared wavelengths allowing a clearer view of the plane and centre of
our Galaxy.

Irregular galaxies, as their name implies, lack a well defined
structure, but usually possess numerous star formation regions and large
amounts of gas and interstellar dust (micron sized particles made up of
carbon and silicon). Galaxies inhabit variously populated regions of
space. The low density regions are well populated by spiral and
irregular galaxies, whilst the denser, rich clusters are dominated by
elliptical galaxies.

An image of Messier 87, a giant elliptical galaxy in the
Virgo cluster.

It has become clear over the last 30 years that extremely dense objects
exist both in our Galaxy and in the centres of many nearby galaxies. In our
Galaxy (and most likely others) small regions of space weighing more than
about 5 of our Suns exist. They consume nearby gas and stars and nothing
ever escapes their grasp. In the centres of large galaxies similar regions
of space exist that also consume stars and gas. However these regions can
weigh as much as several billion (1 billion = 1,000,000,000 or 109)
Suns.

This web site will describe the theory and observations of
these black holes and recent observations of the centres of galaxies
that are providing new ideas about galaxy structure and evolution. The
galaxies with these exotic, extremely massive objects at their centres
may well be called "Hearts of Darkness".

Dark Stars, Black Holes

Shine a torch upwards in the night sky. The light travels along a
straight line then eventually fades, scattered by dust particles in the
air. Travelling at 300,000 kilometres per second light is not hindered
by the gravitational field of the Earth that requires an object to
travel at least 11 kilometres per second to escape its influence. What mass would
Earth need to be to stop the torch light from escaping? Based on
Newton's gravitational laws the Earth would need a mass equivalent to
2100 times that of our Sun. Such a massive Earth would not be a very
hospitable place to live! The intense gravitational field would crush
pre-existing structures. If however we used the existing mass of Earth
and could squeeze Earth into a sphere slightly smaller than a golf ball,
again, light would not escape from its surface.

Theorists from the late 1930s onward predicted that small sized
stellar objects could exist as the final products of stellar evolution.
A "star" with a radius of 5 kilometres would need to weigh about 1.7
times the mass of the Sun to stop light escaping from its surface.
Did such "dark" stars exist?

A schematic view of the formation of a neutron star. A
supernova explosion leaves a massive core of
neutrons behind.

The partial answer to this question was the discovery in 1967 of
radio pulses that came from rotating neutron stars, or pulsars. Pulsars are extremely
small, massive stars made of tightly packed neutrons. They are formed
during a supernova explosion which occurs to high mass stars. Since
their discovery, over one thousand pulsars in the Galaxy have been
discovered. A New Zealand astronomer, Richard Manchester, who works at
the Australian Telescope National Facility, is one of the worlds leading
researchers of pulsars. Whilst neutron stars or pulsars are extremely
massive and small, their largest escape velocity is still only about 80%
of the speed of light. So they are close to being dark stars, but not quite!

People have been thinking about "dark stars" for over two centuries!
In 1783 the Reverend John Michell delivered a paper to the Royal Society
in London announcing that invisible stars may exist if they were
massive enough. The Frenchman Pierre Laplace discussed a similar
phenomenon several years later. Early this century the German astronomer
Karl Schwarzschild succeeded in finding solutions to some outstanding
problems in Einstein's theory of General Relativity, which describes
gravity. Some solutions of Einstein's equations become infinite (called
a singularity) at zero radius. Schwarzschild calculated that a
singularity, could exist at a small radius for a very dense object. For
the Sun this radius would be 3 kilometres. We know this radius nowadays
as the Schwarzschild (or gravitational) radius, and it is that required
by an object so that radiation cannot escape from it. In 1933
astronomers Walter Baade and Fritz Zwicky suggested that the remnant of
a supernova explosion could be a very dense star composed of neutrons.

An artist's impression of a supernova, the explosion of a star.

In 1939 Robert Oppenheimer and
colleagues used quantum theory to determine that stable neutron stars could
exist, and then went further, publishing a paper that would become a classic.
It described massive stars that, once finished thermonuclear burning, would
collapse forever. A physical model for a "dark star" had been found!

A photograph of Supernova 1987A (the bright star lower, right) next
to the Tarantula Nebula in the Large Magellanic Cloud. This was taken by
Alan Gilmore on the 8th of March 1987 using the 60cm reflector at Mount
John University Observatory, Lake Tekapo. The image has been inverted so
that bright features appear dark.

Let's stop for a moment. A problem is looming! How would you detect
an object whose gravitational field is so great that all radiation
(light emitted from a torch is just one type of radiation) cannot
escape from it? The answer is that you cannot observe it directly, but
possibly indirectly, by observing its effect on surrounding objects.

As it turns out, any star greater than 3 solar masses must
eventually form such a "dark star" after thermonuclear reactions have
ceased, since no known source of pressure can support it. These objects
are called "black holes" and this term was first coined by the physicist
John Wheeler.

In 1963 a New Zealand mathematician, Roy Kerr, then working at the
University of Texas, found solutions to the general relativistic field
equations for the case of a rotating star. Since stars rotate, black
holes should rotate, and these solutions were critical in understanding
the space-time effects of spinning black holes. A major breakthrough had
been made. Kerrs solutions showed that as well as having an event
horizon (at the gravitational radius) a spinning black hole had another
important horizon, at a greater radius than the event horizon, called
the static limit. The region between the event horizon and the static
limit is called the ergosphere. Later studies by Penrose, Wheeler,
Bekenstein and Hawking amazingly showed that black holes could emit
radiation from the ergosphere. In general, the smaller a black hole, the
larger the amount of radiation could be emitted. However, even for
stellar mass black holes the rate of radiation is very small, so that
they exist for hundreds of billions of years.

So, are there any black hole candidates? Yes, there exists
strong, indirect, evidence for many. One observational signature is the
rapid variation of high energy X-rays from an object. This variation can be caused by
a binary star system that consists of a black hole orbiting a very large
(supergiant) star. Gas from the supergiant is gravitationally attracted
to the black hole and as the gas approaches it heats up to 1 million
degrees and emits high energy X-rays. A decrease in the strength of
X-rays from the binary system is explained when the black hole goes
behind the supergiant during its orbit. Many such binary systems are
known. One system, Cygnus X-1, in the northern sky constellation Cygnus,
is one of the best candidates for a black hole.

Artist's impression of the Cygnus X-1 binary system, with the
supergiant star on the left, and the black hole surrounded by an
accretion disk of gas, on the right.

Another strong candidate for a black hole is LMC X-1. LMC stands for
Large Magellanic Cloud, a close neighbour galaxy to our Galaxy. LMC X-1
is the strongest source of X-rays in the LMC and it originates from an
unusually energetic binary star system. This source is thought to be a
normal and compact star orbiting each other, similar to the Cygnus X-1
system. The X-rays shining from the system knock electrons off atoms,
causing some atoms to glow noticeably in X-rays. Motion in the binary
system indicates the compact star is probably a black hole, since its
high mass - roughly five times that of our Sun - should be massive
enough to cause even a neutron star to collapse.

An X-ray image of LMC X-1 taken with Röntgensatellit
(ROSAT).

Active Galaxies and Central Energy Sources

Many galaxies possess nuclei that emit vast amounts of radiation.
The amounts can vary from a small fraction to several thousand times
greater than the radiation output of an entire normal host galaxy. In
the 1950s and 1960s radio astronomy provided important clues to the
nature of such galaxies. Powerful radio sources in the sky were found to
be associated with faint elliptical galaxies. Many showed dual lobes of
radio emission on opposite sides of the optical galaxy. The radio
emission was caused by radiation from high velocity, spiralling
electrons in strong magnetic fields. This radiation is called
synchrotron radiation. It was quickly realised that the majority of the
radiation from such galaxies (called active) was not from stellar
sources, but due to this type of high velocity particle emission.

A schematic illustration of synchrotron radiation. Electrons
spiral around magnetic field lines emitting photons of radiation.

Some clues indicated the probable extreme power source of activity
in galaxies. The radio lobes observed on either side of the optical
galaxy were sometimes connected to a small, emission region in the
nucleus of the galaxy via narrow, straight jets. Energy arguments
suggested that the lobes of emission had to be continually replenished
by fast moving electrons. The presence of jets joining the nucleus to
the lobes suggested that something in the small nucleus was the energy
source. Variability in the optical and radio emission of the nucleus on
time scales of hours also suggested a very small energy producing region
(of light hours diameter, similar in size to the Solar System).

Cygnus A: An image obtained with the Very Large Array (VLA) radio
telescope in New Mexico at a wavelength of 6 centimetres. Note the
bright lobes, and narrow jets that point back to the nucleus. The
optical galaxy lies well within the radio lobes, centred on the radio
nucleus.

It is now generally believed that such activity in galaxies is powered
by supermassive objects in their nuclei.

Supermassive objects or black holes?

The presence of supermassive objects in galaxy centres was first
inferred in the late 1970s. Imaging and spectral observations of the
nucleus of the large elliptical galaxy in the Virgo cluster of galaxies,
Messier 87 (or M87, see image above), by Peter Young and Wallace Sargent and
collaborators, suggested the existence of a compact object of 5 billion
solar masses within 300 light years of
the nucleus. This amount of mass is difficult to explain by normal
populations of stars, and many astronomers were convinced that
supermassive black holes (SBHs) easily explained the observations.

Further, the very small size and enormous energy outputs of these
nuclear regions strongly suggest black hole accretion (mass converted to
energy by the extreme gravitational field of the black hole) as the
energy source. Rapid progress has been made recently in the study of central
regions of galaxies by using the high resolution capabilities of the
Hubble Space Telescope (HST) and radio telescopes on Earth. HST is in
orbit around the Earth, and is above the atmosphere that blurs
ground-based optical telescope images.

HST above the Space Shuttle. The gold panels
are solar arrays used to power the telescope. The central white rectangle
is the cover of the Wide Field Planetary Camera 2 instrument that has taken
many high resolution images of galaxy nuclei.

A matter of perspective? The Unified Model

It is now apparent that many features of active galaxies are common.
A model has been put forward that tries to reconcile the differing
properties of activity by assuming that the physical structure in the
nucleus of all active galaxies is similar. The "unified model" assumes
that all active galaxies possess a SBH surrounded by dust in the
shape of a torus (doughnut-like).
Relativistic jets (ie. radio jets) if detected will appear at
right angles to the major axis of the torus.

Variations to the model include the evolutionary status of the SBH
(eg. its mass, possible spin), the type of host galaxy (ie. spiral or
elliptical), the accretion rate of fuel (ie. gas, stars) into the
nuclear (accretion disk + SBH) region, and importantly, the aspect
or orientation of the torus to our line of sight. Such model
variations go a long way to explain the variety of physical properties
seen in active galaxies.

Schematic diagram, not to scale, of the central region of a
Seyfert galaxy illustrating the effect of viewing angle. HBLR/BLR stands
for Hidden/Broad Line Region (high velocity gas) close to the nucleus,
NLR is Narrow Line Region (low velocity gas). Broad spectral
lines are produced by gas clouds with large internal velocities.

By looking along a line of sight into the hole of the torus, we see
the highest velocity gas clouds, nearest to the SBH. Such galaxies are
classified as Seyfert 1, Quasar and Blazar. If
the torus obstructs our direct view, we can only observe lower velocity
gas clouds, further from the SBH, and possibly scattered light from the
nuclear region, and we then detect active galaxies of the Seyfert 2 and
radio galaxy types. In rough order of increasing luminosity the active
galaxies are Seyferts, Radio Galaxies, Blazars and Quasars. It is now
thought that the host galaxies of Seyferts are spirals, and elliptical
galaxies host radio galaxies and quasars although there could be some
overlap. Also, many distant quasars imaged by HST
show peculiar structures that are indicative of interacting or merging
galaxies, suggesting that collisions between galaxies may help to produce
the high luminosity quasars.

An artist's impression, based on HST observations, of a warped, dusty
disk around a suspected SBH in NGC 6251. Perpendicular to the disk is a
jet of relativistic particles ejected along the SBH spin axis.

Nearby Monsters

NGC 4261 - A large, dusty disk

NGC 4261 is a bright elliptical galaxy. It has radio jets extending
well outside the optical galaxy. The HST image shows a large, about 400
light years in diameter, dusty disk slightly inclined to our line of
sight. Note that the radio jets are aligned perpendicularly to the major
axis of the dusty disk (ie. the extended cool region of a torus)
consistent with the unified model. HST spectral observations of gas in
the nucleus suggest a 5 x 108 solar mass SBH.

NGC 4261, Left: A ground based composite optical (white) and radio
(yellow/orange) image. Right: HST image of the galaxy centre showing the
disk of dust. Interestingly, the suspected SBH is some 20 light years
from the geometrical centre of the galaxy. The reason for
this misalignment is unknown.

A word of warning. Even though HST allows us the clearest optical view
of galaxy centres, we do not directly resolve the SBHs or their gaseous
accretion disks. For example, NGC 4261 is approximately 82 million light
years distant, and at that distance, an SBH accretion disk of 1 light
week diameter would span about 1/1000 the size of a HST imaging pixel
element. What we do see in HST images however are the cooler, dusty
disks surrounding the SBH and hot accretion disk. However, the resolving power of
HST does allow important velocity measurements at small distances from
the nucleus, which constrains the mass contained within that distance.

Messier 87 - revisited

M87 is one of the nearest ellipticals that shows signs of activity.
As long ago as 1918 H. D. Curtis discovered an optical "jet" originating
from the nucleus. The optical emission from the jet is also synchrotron
radiation, seen usually as radio emission. The synchrotron
jet occurs at optical
wavelengths when the fast moving electrons are very energetic. M87 is a
powerful radio source (known as 3C 274 and Virgo A) and the radio source
at the nucleus is compact, spanning a diameter of less than 3
light-months.

HST detects a small disk of gas in the nucleus. The disk is
approximately elliptical in shape, and its minor axis is close to the
direction of the optical synchrotron jet. Radial velocity measurements
along the gas disk shows high recession and approach velocities of 500
kilometres per second. A central mass of 2 billion solar masses is
deduced. The authors conclude that the disk of gas is feeding a SBH in
the nucleus, consistent with (but smaller than, by a factor of about
two) the mass inferred from the measurements in the 1970s mentioned
previously.

HST optical observations of M87, showing the nuclear gas disk, and
the spectral signature of rotation. A gas emission line from two regions
of the disk shows a shift in
wavelength indicative of very high relative velocities.

A Mini-Monster in our backyard!

For a number of years evidence has been growing that the centre of
our Galaxy may harbour a SBH. The motions of stars around our Galaxy
centre indicate increased velocities down to very small distances, about
10 light days. The density of matter needed to explain such motions
rules out most alternatives to a SBH.

Left: A near-infrared image of the central 3 light years of the Galaxy
centre. The observation was made with the SHARP I camera on the NTT
telescope at ESO, La Silla, Chile. Right: A contour plot of the image.
The compact radio source Sgr A*, which is associated with
a 3 million solar mass black hole, is just above the central
label "SW".

A radio image of the Galactic Centre at a wavelength of 6 cm,
taken with the VLA. The region is known as Sgr A West
(encompassing Sgr A*) and the emission is due to gas
being heated by nearby hot, young stars.

As in the case of stellar mass black hole systems, we may expect to
detect large amounts of X-rays from an accretion disk around a Galactic
Centre SBH. However, observations have resolved most of the X-ray
emission in the region to a handful of unrelated X-ray binary systems.
The X-ray luminosity of the Galactic Centre is some 7 orders of
magnitude lower than expected for an accretion disk around a 3 million
solar mass SBH. It is therefore possible that if a SBH does reside in
the centre of our Galaxy, it is dormant.

Where to now?

The picture that has emerged is as follows. SBHs are probably a
normal feature of the central regions of bright galaxies that have
spheroidal components (eg. elliptical galaxies, spiral galaxies with a
bulges). SBHs have not been detected in irregular galaxies.
The SBH masses scale roughly with the mass of the host galaxy,
implying a strong link between the growth of the galaxy as a whole, and
the growth of the SBH.

Some fundamental questions remain however. What is the link between
SBHs seen today in relatively nearby and lower luminosity galaxies to
distant, very luminous quasars? Quasars were more populous in the early
universe, and so it is possible that many nearby galaxies were quasars
in their youth, and now harbour relic SBHs that earlier emitted high
(quasar) luminosities. How do SBHs evolve? We also believe that galaxy
mergers were more prevalent at earlier times in the Universe. What part
then do galaxy mergers play in SBH evolution? How would two pre-existing
SBHs behave if their host galaxies merged? Such events may not be
observable by the usual optical, radio or X-ray telescopes, but by the
detection of gravitational waves. A merger of two 107 solar
mass SBHs would radiate energy at a frequency of about 10-4
Hz.

The European Space Agency (ESA) is planning a space-based gravity
wave detector, called Laser Interferometric Space Array (LISA). The
primary objective of the LISA mission is to detect and observe
gravitational waves from massive black holes and galactic binary stars
in the frequency range 10-4 to 10-1 Hz. Useful
measurements in this frequency range cannot be made on the ground
because of the unshieldable background of local gravitational noise.
From recent research and upcoming missions like LISA we are finally
shining some light on these enigmatic hearts of darkness.

An artist's impression of LISA. It consists of six identical spacecraft
forming an equilateral triangle in space with
two closely spaced (200 kilometres) "near" spacecraft at each
vertex. When a gravity wave passes through
the system it causes a strain distortion of space which will
be detected by measuring the fluctuations in separation
between proof masses inside the spacecraft.

Further Reading

Disney, M. 1998, A New Look at Quasars, Scientific American,
June 1998, p36.

Thorne, K. 1995, Black Holes and Time Warps, (Norton: New York).

Acknowledgements

Professor John Hearnshaw and Assoc. Prof. Peter Cottrell of the
University of Canterbury and Dr Ian Bond of the University of Auckland
provided comments on an initial draft of this document.

The DIRBE near-IR image is credited to the NASA COBE project. Alan
Gilmore kindly provided the print of SN 1987A taken by him at Mount John
University Observatory. The X-ray image of LMC X-1 was taken by the
ROSAT, a joint German-U.S.-U.K. project. HST is a cooperative program of
ESA and NASA. Images were obtained from the Space Telescope Science
Institute (STScI; www.stsci.edu) and Office of Public Outreach
(oposite.stsci.edu) web sites. Chris Carilli donated the VLA image of
Cygnus A (Perley, R.A., Dreher, J.W. and Cowan, J.J. 1984, Astrophysical
Journal Letters, 285, L35). Charlene Heisler gave permission to
reproduce the schematic diagram of a Seyfert galaxy (Heisler, C.A.
Publications of the Astronomical Society of Australia, 1998, 15,2,167
and http://www.atnf.csiro.au/pasa/15_2/). The near-IR image and contour
map of the Galactic Center was taken by the Galactic Center Research
Group at the Max-Planck-Institut für extraterrestrische Physik using the
SHARP I camera on the New Technology Telescope at ESO. The VLA 6cm image
of the Galactic Centre is courtesy of Prof. K.Y. Lo, University of
Illinois, Urbana-Champaign, Dept. of Astronomy.

Web Figure References

antwrp.gsfc.nasa.gov/apod/ap950908.html

ftp.seds.org/pub/images/space_art/event04.jpg

antwrp.gsfc.nasa.gov/apod/ap951230.html

ftp.seds.org/pub/images/hubble/wfpcin03.gif

oposite.stsci.edu/pubinfo/pr/97/28/c.html

oposite.stsci.edu/pubinfo/jpeg/ngc4261.jpg

oposite.stsci.edu/pubinfo/pr/94/23.html

oposite.stsci.edu/pubinfo/gif/M87Plot.gif

www.mpe-garching.mpg.de/www-ir/GC/gc.html

therin.ncsa.uiuc.edu/Cyberia/Bima/GalCntr.html#sgr.west.6

www.estec.esa.nl/spdwww/future/html/lisa.html

Author

Glen Mackie isLecturer and Assistant Coordinator Swinburne Astronomy Online (SAO)
in the Centre for Astrophysics and Supercomputing, Swinburne University of Technology.

Acronyms

COBE - Cosmic Background Explorer

DIRBE - Diffuse Infrared Background Experiment

ESA - European Space Agency

ESO - European Southern Observatory

HST - Hubble Space Telescope

LISA - Laser Interferometric Space Array

LMC - Large Magellanic Cloud

NASA - National Aeronautical and Space Agency

NGC - New General Catalogue

NTT - New Technology Telescope

ROSAT - Röntgensatellit

SBH - Supermassive black hole

VLA - Very Large Array

Glossary

Accretion Disk - A disk of matter, usually gas, in orbit around an
object. In a binary
star system gas can be transferred between the stars causing a disk
of gas to form around one star that in some circumstances, is heated to
several million degrees and emits X-ray emission.

Black Hole - A region of space in which the gravitational field is so
strong, neither radiation nor
matter can escape. black holes are thought to form after a star
(greater than or equal to 3 solar masses) undergoes gravitational
collapse. If the star mass is
between 1.4 and 3 solar masses the collapsed remnant will be
a neutron star. A star equal to or less massive than 1.4 solar
masses will collapse to a white dwarf star.

Gravitational Waves - A major disturbance to a gravitational field
will produce gravitational waves. These waves, that travel at the
speed of light, are predicted by
Einstein's General Theory of Relativity. Gravitational radiation
is emitted most strongly in regions of space where gravity is
intense (eg. collapsing stellar cores, neutron stars, black holes).

Hz - The unit of frequency, Hertz. Given in cycles (or number of waves)
per second.

Neutron Star - A small, very dense star comprised primarily
of neutrons. During a supernova explosion the core of the
progenitor star compresses so that protons and electrons
merge into neutrons. Their diameters can be about 20 kilometres
with incredible densities around 1015 grams per cubic
centimetre. Rapidly spinning neutron stars are called pulsars.

Quantum Theory - A theory developed early this century that
describes matter as displaying particle and wave-like properties.

Singularity - A singularity occurs when the solution of equations in
the General Theory of Relativity becomes infinite. This frequently
occurs at a radius of zero, but in the case of Einstein's theory of
gravity, a singularity can also occur near a very massive object
at a very small, finite radius.

Supermassive black hole (SBH) - A black hole that occurs at the
centre of a galaxy. It is uncertain how they form, though their
general physical properties are probably similar to black holes formed
from the collapse of a single, massive star. Supermassive black
holes can have masses as large as several billion solar masses.

Supernova - An explosion of a star. Supernovae can be classified
into two main categories. Type I do not have hydrogen lines in their
spectra, occur in all galaxy types, and are thought to occur when
the white dwarf of a binary star system explodes. Gas transfer
from the companion star to the white dwarf increases its mass above
1.4 solar masses and it explodes. Type II have hydrogen lines in their
spectra and occur in all galaxies except ellipticals. They are thought
to occur when a star of about 8 solar masses finishes thermonuclear
burning.

Thermonuclear Burning - The process in which new elements are formed
in stars by nuclear reactions. Energy is released when lower atomic
weight atoms fuse to form heavier atoms. These reactions only occur
because the temperature in the core of the star is sufficiently large
(about 107 degrees) to give the particles high
velocities so they can interact.

White Dwarf - A star in an advanced stage of stellar evolution.
A star, 1.4 solar masses or smaller, exhausts its sources of
thermonuclear energy, and collapses under its own gravity. The
matter is compressed into a very dense state. A well known white dwarf
star is Sirius B, that has a mass similar to our Sun, but a diameter
about 5 times larger than Earth.